As polyarylene sulfone fibers, porous fibers consisting of the structural units containing ether bondings in their main chains are known. However, these so-called polyethersulfones containing ether bonds in their main chains are thermoplastic and dissolve, for example, in amide-type organic solvents, and although their heat resistance and chemical resistance are excellent, thermoplasticity and amide-type solvent solubility is a limitation thereon.
On the other hand, crystalline polyparaphenylene sulfone powder whose melting point is higher than 500.degree. C. and having heat resistance has been known, but it is practically impossible to melt-mold or solution-mold this powder. There are, however, some examples on film of polyphenylene sulfone in Japanese Laid-Open Patent Publications 92828/1985 and 92829/1985.
Moreover, in Japanese Patent Publication 35370/1984, it is proposed to treat the surface of polyphenylene sulfide fibers with hydrogen peroxide or sodium hypochlorite and so on to make the fibers infusible. However, fibers obtained by this method are very brittle and have low strength. Moreover, defects such as cracks often occur and fibrillation occurs on the treated surface layers of the fibers.